[PATCH 0/2] block: blk-rq-qos: replace static key with atomic bitop

[Nilay Shroff <nilay@linux.ibm.com>]

This patchset replaces the use of a static key in the I/O path (rq_qos_
xxx()) with an atomic queue flag (QUEUE_FLAG_QOS_ENABLED). This change
is made to eliminate a potential deadlock introduced by the use of static
keys in the blk-rq-qos infrastructure, as reported by lockdep during 
blktests block/005[1].

The original static key approach was introduced to avoid unnecessary
dereferencing of q->rq_qos when no blk-rq-qos module (e.g., blk-wbt or
blk-iolatency) is configured. While efficient, enabling a static key at
runtime requires taking cpu_hotplug_lock and jump_label_mutex, which 
becomes problematic if the queue is already frozen — causing a reverse
dependency on ->freeze_lock. This results in a lockdep splat indicating
a potential deadlock.

To resolve this, we now gate q->rq_qos access with a q->queue_flags
bitop (QUEUE_FLAG_QOS_ENABLED), avoiding the static key and the associated
locking altogether.

I compared both static key and atomic bitop implementations using ftrace
function graph tracer over ~50 invocations of rq_qos_issue() while ensuring
blk-wbt/blk-iolatency were disabled (i.e., no QoS functionality). For
easy comparision, I made rq_qos_issue() noinline. The comparision was
made on PowerPC machine.

Static Key (disabled : QoS is not configured):
5d0: 00 00 00 60     nop    # patched in by static key framework (not taken)
5d4: 20 00 80 4e     blr    # return (branch to link register)

Only a nop and blr (branch to link register) are executed — very lightweight.

atomic bitop (QoS is not configured):
5d0: 20 00 23 e9     ld      r9,32(r3)     # load q->queue_flags
5d4: 00 80 29 71     andi.   r9,r9,32768   # check QUEUE_FLAG_QOS_ENABLED (bit 15)
5d8: 20 00 82 4d     beqlr                 # return if bit not set

This performs an ld and and andi. before returning. Slightly more work, 
but q->queue_flags is typically hot in cache during I/O submission.

With Static Key (disabled):
Duration (us): min=0.668 max=0.816 avg≈0.750

With atomic bitop QUEUE_FLAG_QOS_ENABLED (bit not set):
Duration (us): min=0.684 max=0.834 avg≈0.759

As expected, both versions are almost similar in cost. The added latency
from an extra ld and andi. is in the range of ~9ns.

There're two patches in the series. The first patch replaces static key
with QUEUE_FLAG_QOS_ENABLED. The second patch ensures that we disable
the QUEUE_FLAG_QOS_ENABLED when the queue no longer has any associated
rq_qos policies.

As usual, feedback and review comments are welcome!

[1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/

Nilay Shroff (2):
  block: avoid cpu_hotplug_lock depedency on freeze_lock
  block: clear QUEUE_FLAG_QOS_ENABLED in rq_qos_del()

 block/blk-mq-debugfs.c |  1 +
 block/blk-rq-qos.c     |  8 ++++----
 block/blk-rq-qos.h     | 45 +++++++++++++++++++++++++-----------------
 include/linux/blkdev.h |  1 +
 4 files changed, 33 insertions(+), 22 deletions(-)

-- 
2.50.1